1. Field of the Invention
The present invention relates to an X-ray examination apparatus which includes: an X-ray image means for providing an X-ray image composed of pixels, each having a grey value, and a brightness control system which is coupled to the X-ray image means in order to provide the X-ray image means with a brightness control signal of the X-ray image. The present invention also relates to a method for deriving a brightness control value from information contained in an X-ray image.
2. Description of Related Art
Such an apparatus and method are known from EP-A-0 629 105. This document discloses in particular an X-ray examination apparatus having an X-ray image means including an X-ray source, an X-ray image intensifier, a lens structure and a visible image processing device. An auxiliary light detection system of the apparatus forms a brightness control system and provides a brightness control signal which is fed back to the X-ray image means for X-ray image brightness control. In particular brightness and contrast of the X-ray image are being controlled by allowing only certain parts of the visible image to contribute to the brightness control. These parts are called measuring fields, which measuring fields are either selected manually, which is troublesome, or automatically. The prior art application discloses processor controlled automatic selection based on the brightness or grey values of parts in the visible image. The automatic selection raises additional problems in a situation wherein the image shows virtually unabsorbed, so called direct radiation and the object to be examined contains tissues having a low X-ray absorbing, for example lungs, such as in thorax (spine/lungs) images. Discrimination in such an image between parts of the body that may and parts that may not contribute to the brightness control in the X-ray examination apparatus is problematic, also because the position of the lungs in the image may vary.